1. Field of the Invention
This invention relates to cutting tools, and particularly to cutting tools that comprise an elongated boring bar, or tool holder, having one end thereof extending into a socket opening in a support mechanism and the other end equipped with a sharpened cutter insert. The elongated tool holder can be arranged to extend into a machined hole in a workpiece, whereby relative rotation between the tool holder and workpiece enables the cutter insert to machine the hole surface or form a circumferential groove in the hole surface.
2. Description of Prior Developments
It is often desirable to use tungsten carbide as boring bar material for the reason that such material has a relatively high modulus of elasticity and a relatively great stiffness. A boring bar usually has one end received into a socket structure in a supporting boring head or other relatively strong support structure. The other end of the boring bar carries a replaceable cutter insert. The boring bar has a cantilever relationship to the support structure, with the unsupported length of the boring bar often being six inches or more, measured from the mouth of the socket opening. The boring bar should be relatively stiff so that it will have minimal transverse flexure (bending) due to the cutting loads. A boring bar formed of tungsten carbide has a high degree of stiffness and resistance to chatter, caused by vibration.
Unfortunately, carbide is relatively brittle. When a carbide member is formed with external or internal threads, the mating threaded member can break or fracture the thread surfaces on the carbide member when the joint is tightened. In many cases boring bars comprsise a rear shank formed of tungsten carbide and a front shank formed of steel. The two shanks are brazed together to form an elongated boring bar. The tungsten carbide gives the bar good flexural strength, whereas the front steel shank provides a material that can be threaded to receive a screw for retaining the cutter insert.
In many boring bar arrangements, the cutter insert is a flat plate having sharpened side edges extending transversely to the bar longitudinal axis. In many cases the cutting edge of the insert is oriented to lie in a plane coincident with the bar longitudinal axis so that the cutting edge remains on the centerline of the workpiece during the machining operation. Under conventional practice a significant portion of the boring bar front end has to be cut away in order to support the cutter insert in a proper cutting position, i.e. with the cutting face in a plane coincident with the rear centerline. The requirement for removing a portion of the boring bar front end tends to weaken the bar, thus reducing the bar's rigidity and promoting chatter.
U.S. Pat. No. 1,201,614 issued Oct. 17, 1916 to John Oakley, discloses an expanding broaching tool having a rod extending through the body for cutting spiral grooves in the bore of a tubular object.
U.S. Pat. No. 4,642,001, issued Feb. 10, 1987 to Gioffrey Y. Gill, Paul H. Hunter, and Gerard W. Venlet, discloses a spade drill with a replaceable cutting insert. The drill body has grooves for an internal coolant flow.
U.S. Pat. No. 4,848,198, issued Jul. 18, 1989, to Harold J. Royal and Randall E. Deemer, discloses a chip breaking tool holder for replaceable cutting tools. It includes a fluid coupler and seal assembly.
U.S. Pat. No. 4,270,422, issued Jun. 2, 1981 to Ken G. E. Andersson, discloses a cutting tool with a detachable cutting insert. A draw bar holds the insert in positon.
U.S. Pat. No. 4,116,579, issued Sept. 26, 1978 to Martin Norman Hamilton, discloses an end mill tool having an axial bore. A draw bar extends through the bore to pin the cutting insert to the body of the tool.